This is a serious issue. How is it possible for GitHub/Microsoft to charge me for using my own machine as a self-hosted GitHub Actions runner?
They still have to manage state between their servers and yours.
it charges you to use the platform features that enable your use of self-hosted runners
For the same reason they charge you for running Word, even though you're the one who has to write, I guess?
They're charging you for orchestration, log storage, artifact storage, continued development of the runner binary itself and features available to self-hosted machines. What would your own machine do without the runner and service it connects to?